Forget about walls and think "rooms". CA tries to build rooms that can support a flat ceiling and roof. Tweaking individual walls causes issues because the walls are shared by two DIFFERENT rooms with different ceiling heights and CA doesn't know what to do when the ceiling heights are different on opposite sides of the same wall.
Try this exercise: Draw a large room set to 15' height. Now draw four new walls to create a new room inside that room and set the ceiling for tne new room at 10'. Now look at your dollhouse view. You will see that even though the ceiling is set for 10', the walls appear to be 15' tall. WHY? Because these walls are SHARED with the room outside of it, and CA needs to build a flat ceiling so it draws them up to meet the outside room, even though technically they ARE specified structurally as 10' tall (if you have crown mouldings on you will see they are at 10' height.
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Next. Build another room INSIDE that room, double click it to set the ceiling at 10'. CA sees that the adjacent wall is "technically " 10' so allows a 10' room inside it. You have worked around it and now have 10' room.
So this leads to a workaround, using invisible walls to transition from your taller walled sections to your lower walled sections. There may be easier methodss, but this is the only one that comes to mind for me right now.
In your pic above, I would probably take the whole area surrounding the short rooms and set the height at 10', (which would lower the outside walls to 10' also) Then I would draw an invisible perimeter wall BETWEEN the short rooms and the outside wall, then designate that space at 15' to bring the outside walls back up to 15.
